linux-kernel - Re: [PATCH] counter: drop chrdev

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Message-ID: <e8158cd7-fbde-5a9a-f4d9-a863745e3d58@lechnology.com>
Date:   Mon, 18 Oct 2021 11:03:49 -0500
From:   David Lechner <david@...hnology.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] counter: drop chrdev_lock

On 10/18/21 4:14 AM, William Breathitt Gray wrote:
> On Sun, Oct 17, 2021 at 01:55:21PM -0500, David Lechner wrote:
>> diff --git a/drivers/counter/counter-sysfs.c b/drivers/counter/counter-sysfs.c
>> index 1ccd771da25f..7bf8882ff54d 100644
>> --- a/drivers/counter/counter-sysfs.c
>> +++ b/drivers/counter/counter-sysfs.c
>> @@ -796,25 +796,18 @@ static int counter_events_queue_size_write(struct counter_device *counter,
>>   					   u64 val)
>>   {
>>   	DECLARE_KFIFO_PTR(events, struct counter_event);
>> -	int err = 0;
>> -
>> -	/* Ensure chrdev is not opened more than 1 at a time */
>> -	if (!atomic_add_unless(&counter->chrdev_lock, 1, 1))
>> -		return -EBUSY;
>> +	int err;
>>   
>>   	/* Allocate new events queue */
>>   	err = kfifo_alloc(&events, val, GFP_KERNEL);
>>   	if (err)
>> -		goto exit_early;
>> +		return err;
>>   
>>   	/* Swap in new events queue */
>>   	kfifo_free(&counter->events);
>>   	counter->events.kfifo = events.kfifo;
> 
> Do we need to hold the events_lock mutex here for this swap in case
> counter_chrdev_read() is in the middle of reading the kfifo to
> userspace, or do the kfifo macros already protect us from a race
> condition here?
> 
Another possibility might be to disallow changing the size while
events are enabled. Otherwise, we also need to protect against
write after free.

I considered this:

	swap(counter->events.kfifo, events.kfifo);
	kfifo_free(&events);

But I'm not sure that would be safe enough.